Communication systems that utilize coded communication signals are known in the art. One such system is a direct sequence code division multiple access (DS-CDMA) cellular communication system, such as set forth in the Telecommunications Industry Association Interim Standard 95A (TIA/EIA IS-95A) herein after referred to as IS-95A. In accordance with IS-95A, the coded communication signals used in the DS-CDMA system comprise signals that are transmitted in a common 1.25 MHz bandwidth, hence, spread-spectrum, to base sites of the system from communication units, such as mobile or portable radiotelephones, that are communicating in the coverage areas of the base sites. Each 1.25 MHz bandwidth portion of the radio-frequency (RF) spectrum is commonly referred to as a carrier frequency or channel.
In a cellular communication system, a communication link via an RF channel is established between a mobile station, or subscriber, and a source base transceiver station. As a mobile station moves out of range of the source base transceiver station, the signal quality degrades until the communication link is ultimately broken, or the call "dropped". To avoid loss of the communication link resulting from a dropped call, the communication link is shifted from the source base transceiver station to a target base transceiver station, or from a source sector to a target sector within the source base transceiver station coverage area. This process of making the shift is commonly referred to in the cellular communication area as a handoff process.
A handoff can be defined as a change of channel or a change of switch control during a call, either because of degradation of the quality of the RF channel which includes, power level or communication link quality below a certain threshold, or because of the availability of another channel which can allow communication at a lower transmit power, or to prevent a mobile station from grossly exceeding the planned base station boundaries. A handoff may occur during a call in progress (e.g. from a traffic channel to a traffic channel), or during the initial signaling during call set-up.
Handoffs are generally classified into three types; a soft handoff, a softer handoff and a hard handoff. A soft handoff occurs when a mobile communication signal is transferred from a source base transceiver station (BTS) to a target BTS, both BTSs associated with the same base station controller but serving different cell coverage areas. A softer handoff occurs when a mobile communication signal is transferred from a source sector to a target sector, both sectors associated with the same base transceiver station. During a soft and softer handoff, the mobile communication signal is supported simultaneously by both the source and target until the transfer to the target is complete.
A hard handoff occurs when a mobile station is directed to re-tune to a new carrier frequency, and/or the control of resources supporting the mobile communication signal is transferred from a source base station controller, herein referred to as a source central base station controller (CBSC) to a target CBSC. The transfer of a mobile communication signal from a source CBSC to a target CBSC may occur as an inter-CBSC transfer or a inter-system transfer which includes a transfer from a wireless digital communication system to a wireless analog communication system. A hard handoff may occur as an intra-CBSC transfer of a mobile communication signal from a first carrier frequency to a second carrier frequency, if the CBSC has been configured to support more than one carrier frequency.
In digital cellular systems--such as time division multiple access (TDMA) and code division multiple access (CDMA) systems, hard handoffs are generally initiated by a mobile station based on measurements of local pilot signals emanating from neighboring BTSs, wherein each pilot signal has an associated PN offset and represents a specific sector in a BTS coverage area. In CDMA systems, when the energy (Ec/lo) of a pilot signal measurement reaches a threshold, the mobile station initiates a handoff via a pilot strength measurement message (PSMM) sent to the source CBSC, the message containing the pilot signal strength energy measurement. Upon receipt of the PSMM, the source CBSC determines via a database parameter table that a hard handoff is required. The source CBSC then generates a handoff direction message to the target CBSC. The target CBSC allocates target channel resources including the target BTS and sector. The target BTS is also notified of the particular pn long code associated with the mobile station requesting the handoff via the mobile stations electronic serial number (ESN). The source CBSC then sends a handoff direction message to the mobile station. Upon receipt of the handoff direction message, the mobile station retunes to the new pn offset associated with the new target carrier frequency. The target base station then sends a handoff complete message to the target CBSC when its receiver locks onto the mobile station communication signal.
In any type of handoff, the mobile station can recognize only those PN offsets of base station sectors operating within its same carrier frequency. As a result, a mobile communication signal hard handoff from one carrier frequency to another carrier frequency may be initiated by a mobile station upon receipt by the mobile station of a pilot signal from a pilot beacon transmitter. The pilot beacon transmitter facilitates mobile station communication signal hard handoff (i.e. from a first carrier frequency associated with a source BTS sector to a second carrier frequency associated with a target BTS sector) by transmitting a pilot signal on the carrier frequency of the source BTS sector. Since the pilot beacon transmitter operates at a lower power than the BTS sector in which it is co-located, it provides a pilot signal for acquisition by a mobile station entering its coverage area. Upon acquisition of the pilot beacon signal by the mobile station, a PSMM containing the pilot signal energy measurement sent to the source CSBS triggers the appropriate hard handoff sequence.
In congested urban areas where multiple carrier frequencies are utilized, numerous hard handoff seams are created. BTS sectors bordering hard handoff seams require a pilot beacon transmitter to be colocated in each individual BTS sector in order to facilitate mobile station hard handoffs. Consequently, it is conceivable that a BTS sector bordering a hard handoff seam may have as many as ten pilot beacon transmitters, each transmitting on different carrier frequencies. This pilot beacon approach becomes immediately costly in systems with more than three carriers. In addition to hardware costs, many outdoor base stations have very limited space for accessories and have very limited reserves for accessory power consumption and heat dissipation.
Therefore, a need exists for an improved method for determining the necessity of, and facilitating a hard handoff of a mobile station communication signal from a first communication unit, or source base station, to a second communication unit, or target base station, in a wireless communication system.